Friction stir welding (FSW) is a relatively new welding process for joining together parts of materials such as metals, plastics, and other materials that will soften and commingle under applied frictional heat to become integrally connected. A detailed description of the FSW apparatus and process may be found in Patent Publications WO 93/10935 and WO 95/26254; and U.S. Pat. No. 5,460,317, all of which are hereby fully incorporated by reference. One of the useful apparatus for FSW is shown in FIGS. 1A and 1B. As shown, two parts, exemplified by plates 10A' and 10B', are aligned so that the edges of the plates to be welded together are held in direct contact on a backing plate 12'. An FSW tool W has a shoulder 14' at its distal end, and a nonconsumable welding pin 16' extending downward centrally from the shoulder. As the rotating tool W' is brought into contact with the interface between plates 10B' and 10A', the rotating pin 16' is forced into contact with the material of both plates, as shown. The rotation of the pin in the material and rubbing of the shoulder against the upper surface of the material produce a large amount of frictional heating of both the welding tool and the plate interface. This heat softens the material of the plates in the vicinity of the rotating pin and shoulder, causing commingling of material which, upon hardening, forms a weld. The tool is moved longitudinally along the interface between plates 10A' and 10B', thereby forming an elongate weld along the interface between the plates. The welding tool's shoulder 14' prevents softened material from the plates from escaping upward, and forces the material into the weld joint. When the weld is completed, the welding tool is retracted.
Nonconsumable welding pin 16' can have a threaded surface. When a pin with a threaded surface is used, the pin is typically rotated in a negative direction, i.e., in a screwing-out direction, to enhance shearing of workpiece material into particles. This occurs because a threaded pin rotated in a positive direction, i.e., in a screwing-in direction, tends not to generate enough frictional heat to form a satisfactory FSW. A pin rotated in a negative direction, however, encounters greater resistance than a pin without threads or a pin rotated in a screwing-in direction, and thus requires a greater vertical load to drive the pin into workpieces. As the thickness of the workpieces that are welded together increases, the thickness of the weld joint increases. In order to form thicker weld joints, the FSW pin must penetrate more deeply into the workpieces, and a vertical load applied to drive the pin must be increased accordingly. There is a limit, however, to the amount of vertical load that can be applied to the FSW tool, and thus a limit to the depth the pin can be driven into workpieces. A need exists for a friction stir welding tool design that is suitable for penetrating farther into workpieces to form a thicker weld joint without overloading the tool.